In recent years, thin and flat display devices have been needed as display devices in a television, a cellular phone, a digital camera, and the like, and as the display devices satisfying this need, display devices using self-light emitting elements have attracted attention. One of the self-light emitting elements is a light emitting element utilizing electroluminescence (EL), and this light emitting element includes a light emitting material interposed between a pair of electrodes and can provide light emission from the light emitting material by voltage application.
Such a self-light emitting element has advantages over a liquid crystal display element, such as high visibility of the pixels and no need of backlight and is considered suitable for a flat panel display element. Another major advantage of such a light emitting element is that it can be manufactured to be thin and lightweight. In addition, extremely high response speed is also a feature.
Further, such a self-light emitting element can be formed into a film shape; therefore, plane light emission can be easily obtained by forming a large-area element. Since this feature is hard to obtain from a point light source typified by an incandescent lamp or an LED, or a linear light source typified by a fluorescent lamp, the self-light emitting element has high utility as a plane light source which is applicable to a lighting system or the like.
Light emitting elements utilizing electroluminescence are classified according to whether a light emitting material is an organic compound or an inorganic compound. In general, the former is referred to as an organic EL element, the latter as an inorganic EL element.
Inorganic EL elements are classified according to their element structures into a dispersed inorganic EL element and a thin-film inorganic EL element. They are different from each other in that the former includes a light emitting layer in which particles of a light emitting material are dispersed in a binder and the latter includes a light emitting layer formed of a thin film of a phosphor material. However, their mechanisms are common, and light emission is obtained through collision excitation of a base material or a light emitting center by electrons accelerated by a high electric field. For such a reason, a high electric field is necessary for a general inorganic EL element to provide light emission, and it is necessary to apply a voltage of several hundred volts to a light emitting element. For example, a high-luminance blue light emitting inorganic EL element which is necessary for a full-color display has been developed in recent years; however, it requires a drive voltage of 100 V to 200 V (for example, Reference 1: Japanese Journal of Applied Physics, 1999, Vol. 38, pp. L1291-L1292). Therefore, the inorganic EL element consumes much power, and is difficult to be employed for a small-to-medium-sized display, for example, a display of a cellular phone or the like.